Screw propeller or the like



Dec. 1924- 1,5 1 8,502

J. H. W. GILL SCREW PROPELLER OR THE LIKE Filed July 25, 1923 3Sheets-Sheet 1 Fl G. l

J. H. W. GILL SCREW PROPELLER OR THE LIKE I Filed July 25, 1925 3Sheets-Shet 2 m ren/an 1,518,502 J.,H. W. GILL SCREW PROPELLER OR THELIKE Filed July 25, 1923 3 Sheets-Sheet 5 F/G/Z.

WM @TQW M Patented Dec. 9, 1924.

UNITED STATES PATENT OFFICE.

JAMES HERBERT WAINWBIGHT GILL, OF HEACHAM, ENGLAND, ASSIGNOR TO GILLPROPELLER COMPANY LIMITED, OF NORFOLK, ENGLAND, A COMPANY OF GREATBRITAIN.

Application fllcd July 25,

To all whom it may concern:

Be it known that I, J AMEsI-IERBERT WAIN- WRIGHT GILL, a subject of theKing of England, and residing at Heacham, Norfolk, 1n

6 England, have invented certain new and useful Improvements in ScrewPropellers or the Like, of which the following is a specification. s

This invention relates to screw propellers 10 or the like and moreparticularly to rotary impellers for use in axial flow pumps. Theinvention has for its object to provide an improved axial flow impellersimilar to the screw propeller or the like described in the 1 presentapplicants concurrent patentappli cation Serial No. 653,561, July 24,1923. Both the axial flow impeller of the present invention and thescrew propeller or the like of the concurrent application are designedas improvements on the propeller or the like described in the presentapplicants prior United States I of America Letters Patent No. 1454967dated 15th May 1923.. In the screw propeller or the like described inthe specification to that patent a shroud is mounted on or integral withthe tips of the propeller blades, the contour of theirmer surface of theshroud being substantially that of a nozzle designed so as to give tothe fluid stream on which the propeller acts such rate of increasein thevelocity of flow as corresponds to a uniform progressive increase in thed namical equivalent of head (herein referre to b the word head-) perunit axial distance t rough the shroud i. e.

so that there is a constant rate of increase in the square of thevelocity of the fluid stream per unit axial distance through the shroud.

It has been found desirable, however, in

many cases to emplo blades of varying thickness. This varymg thicknessof the blades modifies the nett cross sectional area of the fluidstream, with the result that the propeller designed without allowancesfor blade interference area does not as a whole fulfil the desiredcondition of giving to the fluid stream a substantially uniformprogressive increase in head.

In the screw propeller or the like according to the present inventionallowance is made for the thickness of the blades when SCREW PROPELLEROR THE LIKE.

1923. Serial No. 653,758.

determining the shape of the shroud and the propeller boss, so that thenett cross-sectional area available for the fluid stream flowing throughthe propeller varies substantially propeller. When considering the flowof the whole body of fluid, however, this circumferential component isequivalent to the individual particles changing places with one another,and the phrase effective fluid flow is to be taken to refer to the flowof the whole body of fluid, taking into account only the axial andradial velocity components of the individual particles.

The law governing the variation of crosssectional area may betheoretically stated in.

a number of forms all of which can be shown to be equivalent to oneanother. The necessary condition is that the product of thecross-sectional area at any section and the square root of the axialdistance to the section is constant, this distance being measured alongthe axis from a selected origin thereon. The actual position of theselected origin relative to the inlet section of the propeller in anyparticular case is dependent on the dimensions desired for the inlet andoutlet areas and the axial length of the shroud, these dimensions beingdetermined in accordance wlth practical conslderations,

such for example as the shaft horse-powerand revolution speed of theinstallation and the shape of the hull of the vessel. The governingcondition for the variation of cross-sectional area may also beexpressed in an equivalent form, which is independent of an origin ofreference, in the following manner--that the difference between thereciprocals of the squares of the cross-sectional areas at any twosections is proportional to the axial distance between the sections.

The screw propeller or the like 1s primarily intended for application tosea-going' vessels and is therefore more particularly designed tooperate on liquids.

practical purposes to be incompressible, the quantity of liquid crossingeach transaxial section within the shroud per unit time must beconstant. In other words, the product of the cross-sectional area andthe mean velocity of eflfective fluid flow over: the section isconstant. In this case the above condition of constant product ofcross-sectional area and square root of axial distance is equivalent tothe square of the velocity of effective fluid flow at any section beingpro- Then 2gh=v or how.

The form of nozzle which gives the maximum coefficient of flow is thatin which the rate of change of dynamic head per unit I axial length isconstant, i. e. in which g constant (14: or constant z I I Hence I a 0::1:

Since the volume of liquid passing each slelction per unit time isconstant, it follows t at a-v=constant,

and therefore also that or aw=constant.

area (0. the axial length (8) from inlet to outlet, and theaccelerationto be imparted to the liquid within this length of shroud.

' Since w=constant, thisacceleration determines the outlet area (a,).

Owing to the fact that liquids may be considered for Then, if as, is theaxial distance from the origin to the inlet section,

The blades preferably have an axial increase in effective pitch in thedirection of flow, the rate of increase being inversely proportional tothe rate of variation of the square root of the cross-sectional area ofthe fluid stream. The'blades may be of lenticular section and in thiscase the eifective pitch line lies somewhere between the face and theback of the blades.

' The form of screw propeller which will give the best results is thatwhich is de signed to give a substantially uniform distribution ofthrust over the projected surface of the blades. The rate of chan e ofmomentum of the fluid stream acte on, which results in the axialreaction or thrust is represented by the mass of fluid projected axiallyper unit time multiplied by the absolute velocity imparted thereto. Thismay be expressed mathematically as follows.

fig any transaxial section of the propeller,

t' T=theoretical thrust due to change of momentum of the fluid stream,m=massoffluid per unit volume, a=cross-sectional area of fluid stream,

u=theoretical absolute velocity imparted to the fluid stream by anon-advancing propeller,

P=mean efl'ective pitch of the blade section,

w=axial distance to the section from the selected origin, 1 n=number ofrevolutions per unit time (assumed constant). I Then the expression forthe thrust is T=mau au constant 2 a l. a.

Since the revolution speed is assumed constant, and also uzzPn, itfollows that the pitch P is proportional to u.

Hence l5 0: l a

P a constant.

This gives a theoretical definition of the variation of pitch withrespect to area of cross-section for approximately uniform loading ofthe propeller blades. The foregoing explanation does not take account ofacceleration and true direction of flow, which would vary in accordancewith frictional resistance etc. or of slip at the blades of thepropeller, or of other factors which vary with individual cases, butallowances and corrections should be made for these factors to obtain anaccurately uniform distribution of thrust.

It will be understood that, when pitch, velocity, thrust etc. arementioned in the preceding paragraphs, the mean pitch, mean velocity,mean thrust etc. over the section are meant.

The invention may be carried into practice in various ways, amongstwhich the following may be instanced as a preferred arrangement, someexamples of this arrangement being illustrated in the accompanyingdrawings, in which- Figure 1 is a central section through a constructionof screw propeller.

Figure 2 is an end elevation of the propeller viewed from the outletend.

Figures 3, 4 and 5 are sections through a blade on the lines 3-3, 4-4.and 55 respectively of Figures 1 and 2.

Figure 6 shows in central section an application of the invention to anaxial flow impeller, guide vanes being employed at inlet and' outlet.

Figure 7 shows end elevations in its upper half of the impeller and inits lower half on the left-hand side of the outlet guide vanes and onthe right-hand side ofthe inlet guide vanes.

Figures 8, 9 and 10 are sections through the blades and the guide vaneson the lines 88, 9-9 and 10- 10 respectively of Figures 7 and 8.

Figures 11 and 12 illustrate in end and side elevations respectively amodified arrangement for a screw propeller in which the shroud is builtup in sections, and

Figure 13 shows a section through one of the joints in the constructionshown in Figures 11 and 12.

In Figures 1 to 5 of these drawings the propeller comprises a boss Amounted on the propeller shaft so as to rotate there with, blades Cmounted on the boss A, and a shroud D carried on the tips of the blades,the boss, the blades and the shroud being formed integral with oneanother.

The boss A, which has an after end fairwater B, the blades C and theshroud D are so shaped that the nett available crosssectional area forthe fluid stream is within practical limits inversely proportional tothe square root of the axial distance measured from an origin on theaxis, giving the desired inlet cross-sectional area. and proportionsdepending thereon. The required variatlon in cross-sectional area may beobtamed solely by shaping the inner surface of the shroud after suitableboss and blades able angles. The blades and boss sections are then soproportioned that the nett resulting cross-sectional areas conform asnearly as possible to those of a nozzle designed to give the maximumcoefficient of discharge within the limits determined by the diametersof the ends of the shroud. The incidence of the maximum thickness ofblade section, the effective interference areas of this section and ofother sections parallel thereto, and the corresponding interferenceareas of the boss are taken into account in determining the exact shapeof the shroud and of the boss, so that the nett available area ofcross-section for the fluid stream varies according to the law abo edescribed.

The chain line E on the left of Figure 1 shows what the contour of theinner surface of the shroud would be to give the desired variation incross-sectional area if the blades were assumed to have negligiblethickness. The total interference volume of the blades is thusapproximately equal to the volume enclosed between the surface F and thesu-r face generated by the chain line E. It will be seen that in thisdrawing the bevelled surfaces G G continue the slopes of the ends of thechain line E.

Theoretically the cross-sectional areas should be measured, as has beenstated, on a series of surfaces at all points normal to the flow lines,but in practice a very close approximation can be made by measuring thecross-sectional area on a series of surfaces parallel to that traced outby the leading edges of the blades as they rotate. Tn the exampleillustrated the blades have straight leading edges and are raked back,and in this case the surfaces on which the-cross-sectional areas aremeasured, will be a series of cones.

The blades are of lenticular section, since it is necessary that theyshould be thin at their edges and yet must be thick enough towards themiddle to give the necessary strength. The thickest portion of eac bladeneed not be at the centre of its width but may sometimes be as near theleading edge as one third of the blade width. A satisfactory shape forthe blades is shown in Figures 3, 4 and 5 for the example illus- &

trated, in which the maximum blade thickness is nearer the leading edgeC than the following edge (1 the distances from these edges being about.37 and .63 of the blades width respectively.

The effective pitch of the blades increases in an axial direction fromtheir leading edges to their following edges (taking the normaldirection of rotation). The rate of axial increase in pitch of theblades is inversely proportional to the rate of decrease of the squareroot of the cross-sectional area of the fluid stream, i. e. the productof the square root of the cross-sectional area by the mean pitch overthe section is constant. In calculating the shape of the propeller, itmust be remembered that with lenticular blades the effective pitch line,which is to conform to the selected law, lies between the face and theback of the blades, its actual position depending on the curvature ofthe two surfaces.

In addition to an axial increase in pitch, the blades may in some casesalso be given a radial variation in pitch. In this case the effectivepitch of the blades is made to decrease radially outwards. Thisarrangevment has the effect of giving the quickest rate of flow near theaxis, with the result that dispersion of the thrust column is prevented. The rate of radial variation in pitch is preferably such thatthe curve of the velocity of effective flow plotted against the radialdistance from the axis varies smoothly from a maximum value near theboss to a minimum value at the shroud. With blades as usuallyconstructed, the roots are thicker than the tips, and this in itselfprovides a small radial decrease in effective pitch, but it may beadvantageous in certain cases greatly to accentuate this decrease.

It will usually be desirable, when applying the invention to screwpropellers, to allow the shroud to overhang, that is to project beyondthe blades, more on the outlet than on the inlet side. This is shown inthe example illustrated wherein the bevelled surface G has a greateraxial length than the bevelled surface G. When, however, the inventionis applied to an axial flow impeller, such an overhang is a disadvantagesince the guide vanes, which must be used at inlet and outlet, have tolie fairly close to the blade edges. In this case it is preferable toarrange that the shroud projects little or not at all beyond the bladeedges. This arrangement is shown in Figures 6-10, hereinafter described.in detail. Such an arrangement may also be useful in certain cases withscrew propellers.

The edges of the blades may have any desired contour. Thus thecircumferential projection of the edges on an axial plane (i.

' e. the line of intersection between an axial plane and a surface ofrevolution through the blade edges) may be straight and either radial orinclined, or may be curved. In the latter case the curvature ispreferably such that the blade edges are convex towards the inlet 'side.In the example illustrated in Figures 1 to 5 the blades are raked, thatis the edges are such that a' circumferential projection on an axialplane is straight and inclined back from the inlet side towards the tipsat a small angle to a normal transaxial plane, the edges thus lying onthe surface of a cone. Figure 1 shows a circumferential projection ofthe blade edges on an axial plane rather than a correct view of theedges as seen in a true central section, in order to make theconstruction more clear. This figure also shows on the right-hand side asection along the line of maximum thickness of the blades, in order toillustrate clearly the'change in thickness of the blade from the root tothe tips.

The projection of the blade edges on a transaxial plane may also bestralght and either radial or offset, but is preferably curved, so thatthe blade edges are sickle shaped, i. e. concave towards the normaldirection of rotation, as shown in Figure 2, in which the arrow showsthe normal direction of rotation.

Thus when the particular form of blade to be used has been chosen, theexact contours of the blades, boss and shroud are calculated so that thenett available area of cross-section for the fluid stream variesinversely with the square root of the axial distance from the origin.This is the condition necessary to obtain the maximum coefficient ofdischarge. A propeller constructed according to the present inventiongives a very considerable increase in efficiency over open propellersand also over shrouded propellers in which the shapes of the blades,boss and shroud are not so proportioned as to conform to the laws abovementioned.

To obtain the best results it is found that the ratio of the axiallength of the shroud to the inlet diameter should lie between about 0.20and 0.25. Greater axial lengths merely serve to increase the frictionalresistance to the passage of the fluid and thus reduce the efficiency ofthe propeller, whilst shorter lengths result in insufficient guidance ofthe fluid stream. Again the angle between the main portion F of theshroud and the axis should lie between 9 and 13, the most satisfactoryangle being 11 giving a slope of about 1 in 5. Similarly the anglebetween the axis and the bevelled portion G of the shroud at the inletend should be about 22, and for the bevelled portion G at the outlet endabout 4.

The actual angles of these bevelled surfaces G G depend on the variationof nett cross-sectional area within the portion of the shroudintercepted by the blades, and the slopes are such as to continue thisvariation so that the surfaces continue the curve of the chain line Eshown in Figure 1. Theoretically these surfaces should be curved toconform to the law governing the variation of cross-sectional area, butin actual practice they are made conical, the error thus introducedbeing extremely small.

It has also been found that thereis a limit to the increase in velocityof effective flow, which may be imparted to the fluid stream through theshroud. Thus with a propeller in which the axial lengths of the shroudand blades bear suitable ratios to the leading edge diameter, it isfound that under normal seagoing conditions the increase in effectiveflow velocity from inlet to outlet should not exceed about one-sixth. Inthe example illustrated the percentage increase is about 16.18. Thepercentage increase through the length of the blades is about 11.64.

A propeller constructed according to the proportions specified in thefollowing table has been found to give very good results. In this tablethe diameter of the blades at the leading edge has been taken. as theunit of length and all other lengths are given in terms of this unit.

Blades:

Diameter at leading edge 1.000 Diameter at following edge .940 Axiallength .150 Radius of edges (tra-nsaxial) .500 Maximum thicknessprojected to axis .044 Maximum thickness projected to leading edge tipline .010 Distance of maximum thickness section from leading edge(axially) .054

Shroud Internal diameter at inlet 1.018

These dimensions must be considered as applying to a. propeller havingstraight blade edges, the blades being raked, as in the exampleillustrated, at a slope of 1 in 10. For other slopes allowance must bemade 1n calculating the blade thickness and the blade pitch. Forpractical purposes the elfect of a change of blade rake may be taken asequivalent to a parallel motion effect from The blade section thicknessprojected to axis and to an axial line through the lead ing edge tipwill vary with the width of the blade face, which in turn varies withthe pitch ratio, so that the interference areas of the blades within theshroud remain constant for any pitch ratio. With unit leading edgediameter the mean pitches for the blade faces are represented by thepitch ratios, and these can be so selected as to give a .constant value(.150) to the ratio of axial'blade length to leading edge diameter. Thisnecessitates a variation in the angle subtended by the transaxiallyprojected face of each blade with each variation in pitch ratio. Thusthe angle subtended by each blade has the same ratio to 360 as the axiallength of the blade has to the pitch due to mean diameter of blade. Forinstance in the example illustrated the angle subtended by each blade is40 i. e. of 360, and the mean pitch ratio .is therefore (9 x .150) i. e.1.35 of the leading edge diameter.

The dimensions given in the above table should also be modified inaccordance with the material used in order to obtain the best results.Thus for example when hosphor bronze is emploved instead of cast lI'OIlfor which the proportions are intended) the blades should be somewhatthinner, the maximum thickness projected to axis being about .035instead of .044. The screw propeller illustrated in Figures 1-5 isconstructed according to the dimensions given in the above table.

Figures 6 to 10 show the application of the invention to an axial flowimpeller. In these figures the impeller comprlses a boss H mounted onthe driving shaft, blades J and a shroud K, and is mounted to rotatebetween two sets of fixed guide vanes L and M. The guide vanes L on theinlet side are fixed to or formed integral with a boss N and a shroudingring 0, the outlet vanes M being similarly mounted between a boss P anda shrouding ring Q. The two shrouding rings 0 and Q are separated by adistance piece R surrounding the impeller. The whole assemblyconstitutes an axial flow pump designed to impart a uniformlyprogressive head of flow to the fluid on which it operates.

The construction of the impeller is penerally similar to that of thescrew propeller illustrated in' Figures 1-5. Thus the nett availablearea of cross-section for the fluid stream varies inversely as thesquare root of the axial distance measured from a suitable origin. Themean pitch of the blades increases in an axial dlrection and isinversely proportional to the square root of the cross-sectional area.The effective pitch also referably decreases in a radial direction romthe boss to the tips of the blades.

A detailed description of these featiires has already been given withreference to the screw propeller illustrated in Figures l -5.

.The following descri tion refers in detail- I. only to those features1n which the construo that the edges of the guide vanes L and M tion ofFigures 6-1O differs from that of Fi res 1-5.

lthough curved or raked blade edges should lie close to the edges of theimpeller blades J, the shroud K does not overhang". the blades, i. e.project axially beyond the blade edges. Thus the blades extend axiallyover the full length of the shroud. The number of blades 'may'vary, butin the construction illustrated three blades are employed and the anglesubtended by each blade at the'axis is much wider than in the case ofthe screw propeller. It isfound to be preferable that the blade edgesshould not overlap each other. The blades are again preferably oflenticular section and the line of maximum thickness (shown at S inFigure 7) is more nearly at the centre of the width of the blades thanin the construction of'Figures 1-5. Figure 6 shows on the ri ht-handside a section through one of the; lades, the section being taken alongthe curved line S of maximum thickness. clearly the decrease inthickness of the blades from the roots to the tips. The variation inthickness across the width of the blades is also shown clearlyin-Figures 8, 9 and 10, the chain line in each of these figures beingdrawn through the point of maximum thickness.

The surfaces of the bosses N and P ofing rings 0 and Q continue that ofthe" shroud K. These bosses and shrouding rings, together with the guidevanes L and M, are so shaped that the nett available area ofcross-section for the fluid stream vanes having parallel plane surfaces.Other following edge.

This section shows shapes of vanes may be employed, however, if desired.These vanes serve to guide the fluid in an axial direction to the inletside of the impeller. Any number of vanes may be employed, but thenumber is'preferably not the same as that of the blades of the impellerand is in the case illustrated five. The vanes L extend over the fullaxial length of the shrouding ring 0.

' The outlet guide vanes M have curved surfaces, the slope of thesurfaces near the leading edges (i. e. the edges nearest the impeller)being approximately parallel to the direction of flow of the fluidparticles as they leave the impeller blades, whilst the surfaces at theoutletend are parallel to the axis.- Other slopes may be employed as maybe desirable tosuit particular re-- quirements. in the case illustratedthe leading edge pitch of these vanes is approxi- .into a directionparallel to the axis. The guide vanes M are preferably of lenticularsection and have their line of maximum thickness nearer the leading edgethan the This can be clearly seen from Figures 8, 9 and 10 in which thechain lines pass through the point of maximum thickness. The number ofoutlet vanes M employed is preferably not the same .as l

- the number of impeller blades, four being employed. in the caseillustrated. The vanes shroud The shroud either for a screw propeller orfor an axial flow impeller may be made continuous and mounted on orintegral with the blades or may be made up in sections,

each section being formed by a curved plate mounted on or integral withthe tip of one of the blades. Figures 11-13 show an ar-' rangement inwhich the shroud of a twobladed propeller is divided into two sections.

In these figures the two sections S and T of the shroud are formedintegral respectivelywith the two blades U V, these blades being fixedto the boss W by means of bolts. The shroud sectionsiare bolted Moreoverthe flanges X do not extend from edge to edge of the shroud but only asfar as the planes containin the tips of the leading and following e gesof the blades, the ends of the shroud sections being out straight acrossfrom these planes to the edges of the shroud.

The shroud sections are thus so arranged as to form a practicallycontinuous surface. This arrangement is especially useful for largepropellers particularly for those having separate blades bolted to theboss. In-

Or I

blades carried thereby,

I blades stead of providing a separate section for each blade tip, thesections may be made large enough to extend over and be carried on theends of two or more blades. Thus in the case of a four-bladed propeller,it would be possible to employ two shroud sections, each section beingcarried on the ends of two blades.

The propeller or axial flow impeller constructed according to thepresent invention may be employed, as described, in single form eitherwith or "without guide blades, or two separate sets of blades on thesame boss and within the same shroud may also be employed if desired.The. invention is also applicable to the known arrangement, in which twoseparate shrouded propellers are arranged coaxially but rotating inopposite directions, the inner contours of the two shrouds beingpractically continuous.

It will be understood that the descriptions are given by way of exampleonly, and that modifications may be made in the details of thearrangement without departing from the -scope of the invention.

What I claim as my invention and desire to secure by Letters Patentis 1. An axial flow pump including in combination a rotary impellercomprising a boss, and a shroud mounted on the tips of the blades, a setof fixed guide vanes disposed on one side of the impeller, a fixed bosson which the guide vanes are mounted, and a shrouding ring fixed to thetips of the guide vanes the parts of the pump being so shaped that thenett cross-sectional area available for the fluid stream flowing throughthe impeller and the guide vanes varies substantially in such a mannerthat the difference between the reciprocals of the squares of thecross-sectional areas at any two sections is proportional to the axialdistance between the sections as set forth.

2. An axial flow pump including inv combination a rotary impellercomprising a boss, carried thereby, and a shroud mounted on the tips ofthe blades, two sets of fixed guide vanes disposed respectively on theinlet and outlet sides of the impeller, two fixed bosses each carryingoneset of guide vanes, and two shrouding rings respectively fixed to thetips of the guide vanes blades carried thereby,

of the two sets, the parts of the pump being so shaped that the nettcross-sectional area available for the fluid stream flowing through theimpeller and the guide vanes varies substantially in such a manner thatthe difference between the reciprocals of the squares of thecross-sectional areas at any two sections is proportional to the axialdistance between the sections as set forth.

3. An axial flow pump including in combination a rotary impellercomprising a boss, and a shroud mounted on the tips of the blades. twosets of fixed guide vanes disposed respectively on the inlet and outletsides of the impeller, two fixed bosses each carrying one set of guidevanes, and two shrouding rings respectively fixed to the tips of theguide vanes of the two sets, the parts of the pump being so shaped thatthe nett cross-sectional area available for the fluid stream flowingthrough the impeller and the guide vanes varies substantially in such amanner that the difference between the reciprocals of the squares ofthecross-sectional areas at any two sections is proportional to the axialdistance between the sections, whilst the effective pitch of the bladesof the impeller increases in an axial direction at a rate inverselyproportional to the rate of variation of the square root of thecross-sectional area as set forth. I

4. An axial flow p'ump including in combination a rotary impellercomprising a boss, blades of lenticular section carried by the boss, anda shroud mounted on the tips of the blades and having a substantiallyconical inner surface, a set of fixed guide vanes disposed on one sideof the impeller, a fixed boss on which the guide vanes are mounted, anda shrouding ring fixed to the tips. of the vanes, the parts of the pumpbeing so shaped that the product of the nett cross-sectional areaavailable for the fluid stream flowing through the pump at any sectionand the square root of the axial distance to the sec tion measured froma suitable origin is constant, whilst the effective pitch of theimpeller blades increases in an axial direction -,at a rate inverselyproportional to the rate of variation of the square root of the crosssectional area as set forth.

5. axial flow pump including in combination a rotary impeller comprisinga boss, blades of lenticular section carried by the boss, and a shroudmounted on the tips of the blades and having a substantially conicalinner surface, two sets of fixed guide vanes disposed respectively onthe inlet and outlet sides of the impeller, two fixed bosses eachcarrying one set of guide vanes, and two shrouding rings respectivelyvfixed to the tips of the guide vanes of the two sets, the parts of thepump being so shaped that the product of the nett cross-sectional area I1,a1s,soa

available for the fluid stream flowing of variation of the square rootof the crossthroughthe pump at any section and the sectional area, theeffective pitch of the square root of the axial distance to thesecblades also decreasing radially outwards 10 tion measured from asuitable origin is confrom the boss to the shroud as set' forth.

5 stant, Whilst the efi'ectivc pitch of the im- In testimony whereof Ihave signed my peller blades increases in an axial direction name tothis specification. at a rate inversely proportional to the rate JAMESHERBERT WAINWRIGHT GILL.

